Point-to-multipoint passive optical networks are commonly deployed in telecommunications and cable television local access networks. As shown in FIG. 1A, a typical passive optical network includes an optical line terminal (OLT), often located in a Central Office, and multiple optical network units (ONUs) which are usually located near end-users/subscribers for delivering information (such as voice, data and/or video) services. One or more passive optical splitters are commonly used to distribute signals to/from the OLT from/to the ONUs. Because these optical networks do not require an optical-to-electrical-to-optical conversion at each node, but rather rely on passive optical components, they are considered more reliable than active optical networks.
In the downstream direction (i.e., from the OLT to the ONUs), the passive optical network is a broadcast system; traffic inserted at the OLT is sent to all ONUs. In the upstream direction (i.e., from the ONUs to the OLT), a time division multiple access (TDMA) protocol is frequently used to coordinate the sending of multiple signals to the same OLT point. This is shown generally in FIG. 1B, which illustrates the TDMA data stream received at the OLT.
In general, increasing the upstream communication bandwidth for a fully-loaded passive optical network requires an increase in data transmit speeds. However, higher data speeds typically require more complex and thus expensive electronics for managing packet alignment and performing packet tracking and synchronization. Further, higher data speeds usually require higher speed, higher cost optical transmitters in the ONUs. For these reasons, there are practical limitations on the extent to which bandwidth can be increased in passive optical networks employing current TDMA technology.
As an alternative to TDMA protocols, wavelength division multiplexing (WDM) can be used to provide an ONU with increased upstream bandwidth. However, a system employing WDM requires multiple optical transmitters with closely spaced wavelengths, with each ONU being assigned a unique wavelength. WDM is therefore an expensive solution to bandwidth problems because of the high cost of wavelength specific lasers and multi-channel demultiplexing equipment.